This book revisits the rally-round-the-flag phenomenon in the United States, which is characterized by a sudden and sharp increase in the public approval rating of the sitting US president in response to a war or security crisis. Notably, relatively few wars and security crises have generated rallies. Therefore, the book examines the conditions and processes through which rallies have emerged. The first chapter characterizes the rally phenomenon, explains its political importance, and discusses the book’s broader contribution. The following theoretical discussion (Chapters 2–3) critically assesses the relevance of influential arguments about public opinion for the rally phenomenon and then introduces a novel rally theory. Five empirical chapters feature an extensive comparative investigation of public reactions to US military actions and security crises in the years 1950–2020, through which the author identifies the unique sets of conditions for the emergence of rallies in this period. A sixth empirical chapter analyzes survey data to identify individual-level mechanisms of attitude shifts during rally periods. The findings reveal that rally periods have emerged in the United States in the focal period only under circumstances that lead most Americans to believe it was necessary to take military action to maintain or restore collective honor and gain the respect of other nations. In most rallies, a desire to restore collective honor and prestige stemmed from a preexisting feeling of national humiliation. In response, presidential rhetoric that appealed to national sentiment to justify military action and marked the target around a despised national enemy evoked positive emotions that motivated the majority in the public to rally behind the commander-in-chief. The book concludes by proposing future directions for research on the rally phenomenon in the United States and other countries.